Tissue graft materials have today attained considerable clinical and economic significance. It is estimated that in 1986 $130 million was spent for vascular grafts alone, not including coronary artery bypass grafts. Yet success rates for vascular graft procedures pale in comparison to those of most other surgical procedures. For example, a 5-year cumulative patency of 50% is considered excellent for small diameter vascular grafts. Such low success rates result, in large part, from one or more physical or functional deficiencies in the graft materials currently in clinical use.
Identification of materials suitable for tissue grafts is particularly difficult because such materials must possess a variety of disparate properties. For example, vascular graft materials must not only exhibit mechanical stability under continuous stress, but they also must have porosity adequate for capillarization, compliance similar to that of the host tissue, and high negative Zeta potentials (so as to be nonthrombogenic). Further they should be non-allergenic, non-carcinogenic, and preferably inexpensive to fabricate.
Few, if any, tissue graft materials possess all of the desirable properties. Literature reports of research and development in the area of vascular grafts reflect a significant ongoing effort to overcome the shortcomings common to currently known graft materials.
Both synthetic and autogenous materials have been used for vascular grafts. Among synthetics, expanded polytetrafluoroethylene (PTFE) is a commonly used vascular graft material, particularly for small vessel bypass surgeries. However, expanded PTFE grafts are susceptible to neointimal hyperplasia and late graft thrombosis (e.g., 6-year patency rates of approximately 50% for femoropopliteal bypasses). PTFE grafts are reported to have even lower success rates when used in the venous circulation.
Another synthetic material - Dacron.RTM. - is often used for large diameter vascular graft procedures (e.g., infrarenal aortic grafts). Knitted Dacron.RTM., however, has a relatively high porosity and must be preclotted prior to implantation to avoid extensive hemorrhage. This preclotting procedure is not always practical or successful. Woven Dacron.RTM., while less porous, demonstrates a compliance of only 20% of that found in a normal aorta. Finally, Dacron.RTM. grafts perform poorly in small diameter arteries or veins where blood flow is relatively slow.
One of the more significant problems associated with use of synthetics as tissue graft materials is the fact that synthetic materials have low infection resistance. Infection rates following synthetic graft implantation are associated with a 66% mortality rate. Synthetic materials tend to harbor microorganisms in their interstices and, when contaminated, are extremely refractory to antibacterial therapy. Explantation of infected synthetic grafts is virtually inevitable.
More recently researchers have reported preparation of synthetic skin and blood vessel equivalents utilizing living human cells. See U.S. Pat. Nos. 4,604,346, 4,546,500, 4,539,716, 4,485,097, and 4,485,096.
Among autogenous materials, the saphenous vein, the human umbilical vein, the inverted small intestine, and the radial artery have all been used, but each of these materials has also exhibited significant shortcomings. The saphenous vein may be of an inappropriate size for certain procedures or may be unavailable because of damage by disease. In addition, the saphenous vein may have unacceptable varicosities and suffers from accelerated atherogenesis following "arteriolization." Both the umbilical grafts and the inverted small intestine grafts are plagued by early thrombosis and late aneurysm formation. Finally, the radial artery is of limited utility because it is difficult to harvest and may deteriorate after graft implantation.
It is therefore an object of this invention to provide a tissue graft material which does not exhibit many of the shortcomings associated with many graft materials now being used clinically.
Another object of this invention is to provide a method for preparing a novel tissue graft material from a section of small intestine.
Still another object of this invention is to provide a method for use of a novel multi-purpose tissue graft material in autografting, allografting and heterografting applications.
Yet a further object of this invention is to provide a method for using a novel tissue graft composition for blood vessel replacement.